Ngawha Geothermal Field :Geology, Geophysics, Conceptual Model,
Geochemical Monitoring Trends &Environmental Issues
JOGMEC-GNS Workshop, Tokyo, 2nd June, 2016

Chris Bromley and Greg Bignall
GNS Science, Wairakei ResearchCentre, Taupo, New Zealand
GNS ScienceLocations for New ZealandGeothermal Power Plants
Kawerau

NgawhaAucklandOhaaki
Ngatamariki
Rotokawa
Tauhara
Mokai
Wairakei
2GNS ScienceIntroduction
Ngawha Geothermal Field
OEC units 1&2
Operator : Top Energy
10 MWe (from 1998), 25 MWe (from 2008) – an example of ageothermal project designed and managed to avoid allsignificant surface environmental impacts (ie. no adverse
effects from pressure/temperature changes on hot springs)• ORC (Ormat OEC) binary plants (4 units)• Full reinjection of all fluid produced from 1998 (except ~1.2% CO2 gas initially)• Supplementary surface water injection after 2008 to sustainpressure (to replace all mass, including vented CO2)
• No production cooling so far from reinjectate returns• In 2015, consent granted for expansion by 50 MW• Expect commissioning 2019-2022
GNS ScienceNgawha Expansion Project : pipeline layout

Units 1/2 10 MW ----Units 3/4 15 MW

GNS ScienceNgawhasurface geology,resistivity ‘boundaries’
mapped faults and wells
Few natural earthquakes(GeoNet M>2), & norecorded inducedseismicity (‘quietest’region in New Zealand)so …active tectonic
stresses alone areprobably not sufficient tomaintain permeability inthe host reservoir of
fractured greywacke
GNS ScienceNW-SE Cross-section of Geology

Reservoir is hosted in fractured greywacke basementwith cap-rock of low permeability sediments
GNS ScienceNgawhaGeothermalSystemDC and MT
resistivity‘boundaries’

Key Welllocations:
NG4, NG5
NG9, NG11,NG13, NG18

GNS ScienceMagneto-telluric – resistivity model
3D Leapfrog view

2D MT cross-section
NW-SE(from SKM, 2010)

GNS ScienceGravity
Anomaly heat source ?
Model: ~ 5 x13 km
extent,-300 kg/m3 lower
density,at 3-5 km depth
perhaps coolingrhyolitic pluton ?
~115 km3
If T ~500 oC, heat
content =20 EJ,
If 17 MW surface
heat-loss, age=~40,000 yr ?

Gravity from SKM (2011)
GNS ScienceTemperature-depth profiles

Reservoir

GNS Science

Conceptual model cross-section

GNS Science

Conceptual hydrological model

GNS ScienceResource Areas
NW springsand sinters
Production
expansion
Confirmedresource

GNS ScienceGeochemical Monitoring:Gas emissions halved over 15 years production
2.5Well
NG42NG9NG12
1.5

10.5

001/01/98

01/01/03

01/01/08

01/01/13

Date
GNS ScienceGas ratio (CO2/H2S) has also reduced over 15 years

GNS ScienceGeothermometer temperatures stable for 15 years

GNS ScienceChloride content & Cl/B in total discharge:
show minor changes

GNS Science

Environmental Issues:
Hot spring interference potential(flow-rate, temperature & chemistry)
Subsidence potential
Induced Seismicity potential

GNS ScienceMain Ngawha
Geothermal
Springs : Tiger
bath, Ginns’
bath, Waiarikibath.
Ngawha village
is centre left
and the powerstations are~1km from topleft.

ViewedSouth-west ^Tiger Bath Area

Waiariki Bath(Universal)

Ginns’ Bath(Jubilee)

GNS ScienceNgawha Springhydrology is complex:
4 fluids:
deep Cl, HCO3,
acidSO4+ NH3, &rainfall recharge
High variability before& after productioncommenced,
influenced by low
liquid flows, & highgas fluxes.
Tiger Baths - acid SO4dilutionUniversal (WaiarikiDomain) – HCO3
dilution
Jubilee (Ginn’s Spa)highest Cl (1000-1200ppm)
Christenson, 2003

GNS ScienceJubileeUniversal

Monitoring of Changes in Thermal features

Variable but NO significant response to production

GNS ScienceSubsidence history

GNS ScienceSubsidence model - fit to history

GNS ScienceNgawha subsidence model - predicted results
Predictedsubsidence(1998-2050)
for a 50 MWe
expansion in2020/2023,
using the 3D
Tough2
reservoirmodel tocomputeporo-/thermoelastic
subsidence.

GNS ScienceNgawha subsidence model simulation to 2050
Contour oflevel changefrom thegeothermal
development in
2050 for a 50
MWeexpansion in
2020/2023
using the 3Dmodel(excluding any
backgroundtrend)

GNS ScienceThanks

1998 Ngawha 10 MW OEC 1&2 Power
Station, Top Energy, Northland

Chris Bromley, GNS Science, Wairakei
Acknowledgements: Top Energy, John Burnell, Ed Mroczek,
Mark Simpson, Malcolm Grant, Tricia Scott, & SKM
GNS Science